This invention relates to optical memories for digital data and, more particularly, to a radial tracking and clock control for digital optical disk memories.
As is known, digital disk memories conventionally require a local source of clock frequency information and a radial tracking servo control system. In read only memories, the servo control system is used to precisely control the radial positioning of a focused read light beam so that the read beam is intensity modulated in accordance with the prerecorded serial data on a preselected data track or track sector. A video detector then converts the intensity modulated beam into an electrical video signal. If the recorded data is self clocking, the original digital bit stream is directly available from the video signal, typically as the modulation component of a carrier frequency signal. Otherwise, however, a synchronized clock source is required to supply clock pulses for synchronously sampling the video signal (or, more specifically, the demodulated video signal) at the appropriate rate to recover the original digital bit stream. Direct read after write digital optical disk memories have additional requirements because of the write function. In particular, a clock pulse source is required in conjunction with the input data so that such data is converted while being read out into a digital bit stream having a predetermined bit rate, and the radial tracking servo control system is needed for precisely controlling the radial positioning of an intensity modulated focused write light beam so that the digital data is serially recorded on a preselected data track or sector.
Heretofore, the need for clock frequency information and for radial tracking servo control have generally been treated as separate, unrelated requirements. For example, in U.S. Pat. No. 4,094,010 it is suggested that a groove be "burned" into the disk for radial tracking purposes and that an external clock be used to supply the clock pulses needed for writing and reading. Another relevant reference is Kenney et al., "An Optical Disk Replaces 25 Mag Tapes", IEEE Spectrum, February 1979, p. 33, where it is suggested (at p. 37) that digital data be recorded in self clocking form, thereby avoiding the need for synchronizing an external clock while data is being read from the disk.
As will be appreciated, the requirement that the external clock be synchronized for a read mode operation is one of the basic disadvantages of the digital optical disk memory described in the above-identified patent. Likewise, the memory described in the above-identified article suffers from the data density limitations which are inherent in recording digital data in self clocking form.